Flame detection devices, or burner scanners, are well known in the art for monitoring burner flames as part of maintaining safe operating conditions. For example, if fuel such as oil or gas is continuously delivered to a burner even though the flame has failed to ignite or has become extinguished, the results can be an undesirable explosion. The common technique of avoiding such potentially severe consequences is to use an optical scanner, that "watches" the flame and triggers a flame relay for sounding an alarm and cutting off the fuel supply if there comes a time when the scanner fails to "see" a flame.
Flame detection devices using two different sensors are known in the art.
U.S. Pat. No. 3,476,945 to Golden et al. shows a flame detector incorporating two characteristically different elements, each of which responds to the flame characteristics of a specific fuel. Golden et al. describes the use of an ultraviolet radiation detector and also a detector primarily sensitive to visible and near infrared radiation.
Another dual detector flame sensor is shown in U.S. Pat. No. 4,370,557 to Axmark et al., wherein the outputs of a visible light sensor and an infrared sensor are amplified and then summed in an adder. The output from the adder is then passed through a variety of amplifiers and rectifiers to an indicator or alarm.
U.S. Pat. No. 3,665,440 to McMenamin shows a fire detector using ultraviolet and infrared sensors, wherein the respective outputs of the two sensors are fed into a false alarm inhibit circuit. There is no output signal from this inhibit circuit unless the ultraviolet detector is not detecting ultraviolet radiation, or the ultraviolet radiation being detected is at a frequency outside the passing band of a band pass amplifier. The system is designed to inhibit the setting off of false alarms by, for example, stray illumination from a match or cigarette lighter flame.
U.S. Pat. No. 3,940,753 to Muller shows a flame detection device using at least two photoelectric sensors, which are sensitive to different spectral ranges of incident light. The relationship between the AC components of the two sensed output signals is evaluated to determine whether a "flame present" signal should be provided.
Other dual sensor devices include the flame detector and electrical detection circuit of U.S. Pat. No. 3,716,717 to Scheidweiler, et al. and the flame detection system of U.S. Pat. No. 3,967,255 to Oliver et al.
However, these prior art devices are limited in their usefulness for different burner types and arrangements, different fuels, different furnace capacities, and applications wherein susceptibility to power line noise may be particularly acute.